The present invention relates to the field of optical switching. The use of traveling-wave optical amplifiers for 2.times.2 switching was demonstrated by Ikeda, see M. Ikeda, "Proposal of laser diode integrated-optical matrix switches", Trans. IECE Japan, vol. E69, 1072-1074 (Oct. 10, 1986), who fashioned his integrated-optical device from laser-like InGaAsP/InP ridged waveguides. Also see U.S. Pat. No. 4,521,069 to Ikeda.
A number of problems became evident with his disclosed devices. First, the small radius of curvature in the curved guides introduced a large radiation loss in those channels. Second, the optical paths do not have the same length, which leads to different biasing requirements for the cross and bar states. Third, the choice of bulk material is not optimum because of noise problems discussed below, and because the optical gain in a waveguide per unit of injection current is smaller in bulk than in multiple-quantum-well (MQW) material. Lossless operation (zero dB insertion loss) is an important goal for the switches of the present invention.
In U.S. Pat. No. 4,778,235, the inventor, Fujiwara discloses switches which utilize the lateral spreading or fanning out of light, to produce the desired light losses at selected legs of the switch. See FIG. 6 in particular, and col. 7. His switches are "gain guided" so that at zero bias, portions of his waveguide "disappears". In sharp contrast, the switches of the present invention employ index-guided-channel waveguides, having strip-loaded ribs, buried inverted ribs, or other embedded corrugations, producing a lateral index step for horizontal confinement of light as well as the vertical confinement. In the present invention, band-to-band optical absorption in the waveguide at zero or reverse bias, produces the desired optical attenuation, in contrast with the lateral fanning-out of light which occurs in waveguide portions of all of Fujiwara's devices. As a result, our on-off light contrast ratio will be 1000/1 or more in contrast with Fujiwara result, estimated at 100/1 or less which will thus produce more crosstalk than switches of the present invention. Regarding his directional coupler of FIGS. 3 and 4, one of the waveguides will disappear so that he doesn't teach a true 2.times.2 directional coupler switch where both data channels are preserved at both outputs, for both the bar and cross states, and thus light-modulated data would be lost.
Although he suggests utilizing refractive index changes and gain differences in what he calls a directional coupler, he cannot preserve both data channels, as one waveguide disappears. That is, he never has two index-guided channels for a true 2.times.2 interferometric directional coupler.
U.S. Pat. No. 4,737,003 to Matsumura teaches quantum well switching devices with carrier injection at cross-over guide portions. These however are of the deflection or refraction type, as is U.S. Pat. No. 4,784,451 to Nakamura et al, and carrier injection to produce optical gain is not taught therein.